1. Field of the Invention
The present invention relates to thermostable mineral compositions, and more specifically, to thermostable mineral compositions which are soluble in biological fluids. Even more specifically, the present invention relates to mineral fibres formed from the mineral compositions and relates to mineral fibre insulation and a mineral fibre plant growing medium made from these compositions.
2. The Prior Art
Mineral fibre insulation is widely used and has been a commercial product for a long period of time. The insulation products are made from mineral raw materials such as rock or slag which are melted and spun into fibres that a binder holds together. The binder is usually a phenol-formaldehyde resin or a urea-modified phenolformaldehyde resin.
It is well known that mineral fibre insulation products are advantageous as compared to glass fibre insulation products in their higher fire resistance, i.e., they have an excellent thermostability. Typically, glass wool withstands temperatures up to around 650.degree. C., whereas mineral wool is capable of withstanding temperatures up to about 1000.degree. C. It is highly desirable to maintain or even increase this excellent property in any modification of the hitherto known mineral fibre products.
Recently, more attention has been given to health issues in connection with various fibrous material, including insulation wool fibres. It is well known that inhalation of certain types of fibres such as asbestos fibres may lead to respiratory diseases, including lung cancer. It is believed that an important factor is the ability of the asbestos fibres to remain in the lung for extended periods of time. Although there has not yet been provided any evidence of manmade fibre being the cause of respiratory or other diseases in man, it is desirable to provide mineral fibres with an increased dissolution rate in biological fluids, since it is expected that such fibres will have a considerably shorter half-time in the lung upon inhalation.
The concern for possible health effects of man-made vitreous fibres (MMVF) has been the reason for a number of investigations in the recent years. It is believed that besides the fibre dimensions, also the time of residence in the lung may be an important parameter for causing disease. The residence time is influenced by the physical clearance of the fibres from the lung and by the rate of dissolution of the fibres.
The rate of dissolution of fibres may be assessed in different ways. In vitro measurements have been applied, subjecting fibres to artificial, physiological solutions (Gamble's solution, modified according to the disclosure in Scholze, H. Conradt.: An in vitro study of the chemical durability of siliceous fibres. Ann. Occ. Hyg. 31, p. 683-692, (1987)), believed to resemble the conditions in the lung fluids. The liquids used are all characterised by having a pH of 7.4-7.8. It is known from Carr, Ian: The Macrophage - A Review of Ultrastructure and Function. Academic Press, (1973), that the pH in the macrophages is different, more acidic, than that of the lung fluid, resembled by the normally used liquids.
Recent published measurements of fibre durability, including in vivo measurements of the fibre dissolution in rat lungs indicate that this difference in pH may account for different rates of dissolution, and it is demonstrated that the investigated fibres: when sufficiently short, may be engulfed by the macrophages and this might explain the lower rate of dissolution observed for shorter glass wool fibres.
WO 89/12032 discloses inorganic mineral fibre compositions among which some have passed the ASTM E-119 two hour fire test as well as exhibit low durabilities in physiological saline solutions, i.e. have high dissolution rates therein. The components of the disclosed compositions may vary considerably. However, all the disclosed compositions are made from pure metal oxides or from less pure raw materials with addition of pure oxides which make the disclosed compositions very costly.